1. FIELD OF THE INVENTION
The present invention relates to an organic thin film having an electric or optical function and a method for producing the same, and more particularly to an organic thin film composed of organic molecules which are in good orientation and a method for producing the same.
Further, the present invention relates to a liquid crystal display device, and more particularly to a thin or flexible liquid crystal display device composed nearly alone of organic substances.
Still further, the present invention relates to an orientation-controlling film for use in liquid crystal display devices, and particularly to a liquid crystal display device in which the above organic thin film is used as a liquid crystal orientation-controlling film suitable for use in nematic and smectic liquid crystal display devices.
2. DESCRIPTION OF RELATED ART
Recently, studies on functional organic thin films are being actively made with a mind turned to future electronic devices, the so-called "molecular electronics". These studies are intended to apply electronic, optical and chemical functions inherent to organic molecules to electronics and optoelectronics materials, energy conversion materials and the like. In order to develop these various functions effectively in the form of a molecular assembly, particularly a thin film, it is said to be indispensable to highly control the structure of the molecular assembly, in other words, to control the orientation and arrangement of the constituent molecules. The conventionally known thin film-making techniques include dry-type thin film-making methods such as a vacuum evaporation method, molecular beam epitaxy method, cluster ion beam method, ion beam evaporation method, ion plating method, photo CVD method, plasma CVD method, etc., and wet-type thin film-making methods such as a Langmuir.Blodgett (LB) method, dipping method, spraying method, spin-coating method, casting method, electro-deposition method, etc. Particularly, the LB method is a technique which is recently re-appreciated because of its ability to form a multilayer film having a periodic order of molecular order in the deposition direction of monomolecular films. At present, organic thin films composed of organic molecules in controlled orientation and arrangement are being produced by these methods, and the thin films obtained are being characterized. Since, however, most of the organic thin films produced by the above various methods are a polycrystalline thin film which is low in in-plane orientational order and difficult to develop good functions, various devices are being made in order to improve these defects. For example, the following methods have been thought out: In the LB method, a method of moving a substrate up and down to induce the flow orientation of organic molecules or molecular aggregates in the monomolecular film, thereby improving the in-plane orientational order of the molecules; and in the vacuum evaporation method, a method of promoting epitaxial growth using the cleavage plane of an alkali halide as the substrate to which a vacuum evaporation is applied, and a method of producing a thin film of oriented polydiacetylene by rubbing the substrate.
The foregoing conventional techniques are described in Japanese Patent Application Kokai No. 64-18 470, J. Le Moigne et al., J. Chem. Phys., 88, 6647 (1988) and M. Era et al., Polymer Preprints. Jpn., 37, 3279 (1988).
Even the organic thin films which are produced by the foregoing various methods and said to be improved in the in-plane orientational order are not still said to be satisfactory in the orientational order and its completeness, and problems are still left. For example, when the flow orientation effect is used in the LB method, the degree of in-plane orientational order is non-uniform. Also, in the method wherein the cleavage plane of an alkali halide is used as the substrate to which the vacuum evaporation is applied, there are defects such as generation of grain boundary and poor stacking which are thought to be due to non-matching of lattice constants. Further, the organic thin film-making methods now in use are difficult in obtaining a thin film having an area as large as about several tens cm.sup.2 and a uniform in-plane molecular orientation.
The present inventors have studied to solve the foregoing problems and defects, and as a result, have found that a uniform and highly oriented thin film is obtained by the method which comprises forming a monomolecular film or such a film at a gas/liquid interface containing a polymer having a particular functional group as a main film-forming substance, copressing the formed monomolecular film or such a film on the liquid surface unidirectionally or in anti-parallel direction facing each other to form an organic thin film at the gas/liquid interface in which the film-forming molecules have been oriented in a particular direction in the surface of the film and then transferring the thin film to a solid substrate by a horizontal lifting method or horizontal immersing method as well as by the method of forming a thin film which comprises applying various deposition techniques to the thin film produced by the above method.
As is shown in FIG. 1, the conventional liquid crystal display device is composed of polarizers 6 supported by glass substrates 5, spacers 10 for maintaining the thickness of the liquid crystal layer 9 constant, orientation-controlling films 8 for orienting the liquid crystal, transparent electrodes or semiconductor devices 7 for driving the liquid crystal and glass substrates 5 for holding the whole liquid crystal cell. Such the conventional liquid crystal cell has problems that they are thick and heavy. In order to overcome these defects, however, there is reported a technique of reducing the weight of the cell or making it flexible by replacing the upper and lower glass substrates of the cell by plastic films or plastic plates (Japanese Patent Application Kokai No. 53-19042).
In the above conventional technique, however, a sufficient consideration is not given to making the liquid crystal cell flexible, so that there are problems to be solved for realizing flexible liquid crystal display devices. Concretely speaking, consideration is not fully given to the glass substrates supporting a polarizer and an analyzer held in the liquid crystal cell, and also there is no sufficient device to keep the thickness of the liquid crystal layer constant.
The present inventors have found that by using the method of transferring a monomolecular film or such a film spread on a gas/liquid interface successively to a solid substrate to form an organic thin film, a flexible and/or thin liquid crystal display device composed nearly alone of organic substances can be produced in which a transparent electrode(s) or semiconductor device(s), a polarizer, an analyzer, an orientation-controlling film and a liquid crystal layer of uniform thickness are disposed in layer on a transparent polymer film.
For obtaining a good display quality of liquid crystal display devices, it is important to cause the liquid crystal molecules to take a uniform orientation by forming an orientation-controlling film in the inside of the devices. Therefore, a large number of studies on the orientation-controlling film have been made. As typical examples of such the film, there is known an organic orientation-controlling film made to acquire an orientation-controlling ability by applying the rubbing treatment to the film of an organic polymer (e.g. polyimide) (Japanese Patent Application Kokai No. 50-83051 and No. 51-65960), and this film has been put to practical use. Recently, further, it is proposed to use as the orientation-controlling film a polyimide film deposited in layer on a substrate by the LB method (Japanese Patent Application Kokai No. 62-209415, No. 62-211617 and No. 62-215928).
Recently, however, it has become clear that the afore-mentioned orientation-controlling film has some problems when viewed from the standpoint of a liquid crystal device-manufacturing process.
For example, the common orientation-controlling polyimide film, which is produced by the method of applying a polyimide precursor solution to a substrate by means such as a spinner method, dipping method, printing method, etc., has a problem that a sufficiently uniform film thickness is not obtained when the formation of a film of 1500 .ANG. or less in thickness is aimed at by this method. Non-uniformity in thickness of the orientation-controlling film causes non-uniformity in the threshold voltage of a liquid crystal display device, as a result of which unevenness of display sometimes occurs.
When organic polymer films formed by the above means are used as the orientation-controlling film, a treatment of rubbing the film surface with cloth, called "rubbing", is applied. This treatment sometimes generates static electricity on the film or stains the film surface. The static electricity generated on the orientation-controlling film sometimes breaks the transparent electrode on the substrate, in which case non-lighted parts appear in the liquid crystal display device.
In an active matrix liquid crystal device, the static electricity generated by the rubbing treatment sometimes breaks its thin film transistor (TFT) or changes the switching characteristics of the transistor. When the surface of the orientation-controlling film is stained due to the rubbing treatment, the frequency dependency of the threshold voltage of the device becomes non-uniform.
With an increase in the size of the substrate, it becomes difficult to control a load of rubbing on the whole substrate, so that the large-sized substrate is sometimes scratched by the rubbing.
When a polyimide precursor is applied or deposited on the substrate by either of the spinner method, dipping method, printing method or LB method, the precursor must be imidated by heating or chemicals treatment. Such the treatment is not desirable in principle for the substrate loaded with TFT.